The CNC Saga, Part 1

I´m going to hold off on recounting the four day trip east and south to Saipina and Pasorapa, so I can tell the stories good and proper. What we did was go around conducting surveys on stove usage, and we got around 50 although I mostly just took pictures as my Spanish isn´t up to snuff. But I did call home from a phone booth and get some pretty awesome pictures. It´s the little triumphs that count.

What I want to do now is talk a little bit about the CNC plasma cutter I´ve been working with for the past two weeks. I call the story "The CNC saga" because the story is already long, is epic at least from my standpoint, and will probably have a tragic ending. I´ve been working with a Bolivian engineer to try to get this damn thing to work for two weeks, and one problem after another keeps creeping up.

CNC stands for Computer Numerical Control, which uses a programming language called G code to automatically control what a machine tool does. In this case, the machine tool is a plasma cutter, a device which uses an electric current to superheat gas into plasma and cut through sheet metal. In this case, we´re cutting through 1/16" sheets of steel to make tapas, or the tops of rocket stoves. The plasma torch is mounted on a gantry which can travel in the x and y directions on rails using stepper motors. It´s a really cool machine, actually a kind of robot in that it is a machine which does the machining that a human operator might do, only with far greater precision and speed.

In fact, robotics was the subject which got me into engineering, in idyllic days before high school when men were men, boys were boys, and you could still buy very good and cheap canned root beer at Meijer. I worked with my friends Steve and Adam in a basement, designing and building a lego robot in Steve´s basement for the FIRST Lego League competition. We carried the state of Ohio two years running. Back then we called the robot "Free Willy" because we swore it had a will of its own. It wouldn´t do anything we programmed it to do. Now, when life has carried me to Bolivia, I find those words prophetic because this robot won´t do what we tell it to. The difference is that this robot costs thirty thousand dollars and can actually kill you. The plasma torch draws 40 amps of current and generates temperatures hot enough to vaporize steel.

Where the CNC plasma cutter fits into CEDESOL´s operations is that we´re trying to industrialize the production of rocket stoves and solar ovens. The idea is to have them in just about every village in Bolivia, which was partially the motivation for the trip to Saipina. We can use the cutter to rapidly cut out metal parts for stoves, and we want to do several thousand a month. Ideally, you can program the cutter to do what you want, put sheet metal under it, press "go," and the cutter will give you twelve stove tops like cutting out cookies on a baking sheet.

However, there are always hang-ups. For one thing, when the gantry travels along the rails it has a tendency to hang up on one side, stopping momentarily. This throws the cutter navigation off so that it starts cutting the second tapa over where the first tapa should be, or starts cutting the screw holes in the wrong place. Factor in the price of sheet metal these days, lost time and energy, and these mistakes start to look very expensive. Secondly, the sheet metal doesn´t lie flat, but bubbles up in the middle. The CNC system is equipped with an automatic height control which is supposed to compensate for material bubbling by sensing the torch voltage and jogging the torch up or down, but for some reason that doesn´t work. So the torch eventually hits the material and hangs up as well. The upshot of it is is that we have a program that works cutting three tapas at a time when we should be able to cut twelve, and two of us have to baby the machine the whole way in case something goes wrong. The whole cutting process almost takes longer than it would to just cut out the tapas manually. We´ve got about ninety right now, and cut sixty yesterday, but that took all day.

Hopefully the work of the afternoon will go better we can solve the motor problem, and I can calibrate the height control to avoid the torch catching the material. Hopefully.

Hasta Luego,
Drew